1. Field of the Invention
The present invention relates to the enhancement of traditional cable location methods by a fusion of electromagnetic sensors and/or ground penetrating radar sensors with inertial sensors in a state-space framework.
2. Discussion of Related Art
Underground pipe and cable locators (sometimes termed line locators) based on electromagnetic (EM) methods have existed for many years, are well known, and are relatively easy to use. EM line location usually involves a conductive target cable or pipe so that the target cable or pipe can carry an AC current. Ground-penetrating radar (GPR) may be utilized to locate conducting or nonconducting pipes and cables. However, the widespread use of GPR has (in part) been limited by the difficulties in interpretation of the typically graphical output of such systems, which are much more complex than EM locators.
EM line locator systems typically include a mobile, handheld receiver and a transmitter. The transmitter can be coupled to a target conductor, either by direct electrical connection or through induction, to provide a current signal on the target conductor. The receiver detects and processes signals resulting from the electromagnetic field generated at the target conductor as a result of the current signal. The signal detected at the line locator receiver can be a continuous wave sinusoidal signal related to the target signal on the target conductor generated by the transmitter.
The transmitter is often physically separated from the receiver, with a typical separation distance of several meters or in some cases up to many kilometers. The transmitter couples the current signal, whose frequency can be user selected from a selectable set of frequencies, to the target conductor. The frequency of the current signal applied to the target conductor, which can depend on conductor type, can be referred to as the active locate frequency. The target conductor generates an electromagnetic field at the active locate frequency in response to the current signal.
In EM line location systems, the signal strength parameter determined from detection of the electromagnetic field provides a basis for derived quantities of the current signal (i.e., the AC current in the targeted conductor), position of the line locator receiver relative to the center of the conductor, depth of the conductor from the line locator receiver, and can also be used as the input to a peak or null indicator (depending on the orientation of the magnetic field to which the detector is sensitive).
For detection of cables or pipes laying in a continuous path (e.g., buried in a trench or occupying an underground conduit extending over some distance), an assumption is often made that the induced magnetic field is concentric around the cable and that signal strength is dependent only on the local ground conductivity, the depth and horizontal position of the target cable, and the magnitude of AC current flowing in the cable. When this is the case, the electromagnetic field at the detector of the line locator, on which the signal strength depends, is said to be free of distortion.
Nearly all EM locators present a “peak” indication that results from a horizontally oriented coil, with axis orthogonal to the direction of the cable that has a maximum deflection over the presumed centerline of the cable, assuming the ideal undistorted field. Some locating systems also present a “null” output from a vertically oriented coil, which has a minimum at the same position, again making the same ideal assumption about the magnetic field. Yet other locating systems create a null response by horizontally aligning two coils and subtracting the output of one from the other. The intent in all these coil configurations is to create coil responses to the induced magnetic field that lead to the deduction of the target conductor position.
Often in a crowded underground utility environment of metallic pipes and cables, coupling of signals at the active locating frequency from the target conductor to other adjacent underground conductors can occur. These conductors (lines) are not intended to be tracked by the line location system, but coupling of currents from the target conductor to those neighboring conductors through various means (resistive, inductive, or capacitive), termed “bleedover,” can lead a line locator astray such that the operator of the line location system ceases tracking the targeted conductor (e.g., pipe or cable of interest) and instead begins following an adjacent line. In some cases, there may be bias in the targeted conductor's estimated centerline as a result of distortion due to bleedover.
A disadvantage with relying solely on an EM line locator device for determining the location and depth of a target conductor is that the locator may fail to identify and distinguish among various utility conduits and conductors. EM line locator devices especially may fail to locate non-conductive lines, such as, for example, gas lines, fiber optic lines and plastic water lines when those lines are not marked with trace wires.
Ground penetrating radar (GPR) systems have also been used for utility locating applications. However, such systems present to the user a fairly complex grey-scale or color image of the radar pulse echo amplitude in a 2-D map of depth vs. horizontal ground position. This type of display is useful in survey and mapping applications, but provides far too much information for the ordinary user trained in line location techniques. In conventional GPR systems, the results that signify the presence of underground utility lines are often not available until after post processing of the image data collected on an external computer.
Conventional GPR systems focus on collection of data, and not the detection and tracking in real-time of an individual utility line and therefore provide highly complex displays of that data. Conventional EM line locator user interfaces are designed with easy; displays for infrequent use. The locate technician must be able to pick up the instrument after sporadic use and be able to immediately track a specific line though an unfamiliar area. The line location is often marked with paint in response to the indications presented on the display. The painted location should have accuracy within accepted (and sometimes legal) guidelines. Images of the data that are presented to the user are constantly changing as the user is walking the locator down the line. Further, features presented in the images are often inconsistent and unrecognizable when these maps are presented in real-time to the user. Using conventional methods without off-line image processing, centerline errors of a few centimeters are attainable with EM line locators but are not achievable with a GPR line locator.
Depth of the target utility line is an important parameter in any locating task. The locate operation often precedes a digging operation and the result of the locate determines how deep a backhoe operator can dig without impacting the utility line. To achieve acceptable depth accuracy, conventional GPR systems can be calibrated by locating a known underground object on the same test site (because soil conditions are generally identical between the calibration and the locate), and then digging down to the object to determine the exact depth. Without an absolute depth confirmation, GPR systems rely on a user-specified choice of the dielectric constant of the soil at the test site. Determining the dielectric constant of the soil is often difficult at best, resulting in depth determinations that are subject to 20% tolerances in this scenario. Further, cable and pipe locate operations typically involve a manual painting operation to indicate the location of the line. The digging crew later uses this marking as a guide to avoid damaging the cable while excavating. Unfortunately, markings are not permanent and therefore can not be relied on for future locates at the same site.
Therefore, there is a need for a dual-mode line locator that simultaneously offers both metallic (electromagnetic) and non-metallic (ground penetrating radar) location methods, particularly for the real-time location of the line's centerline and depth. There is a further need to process radar range data obtained from a GPR locate and present a simple user interface display, enabling an operator familiar with conventional EM line tracing methods to trace non-conductive lines with a GPR locator. There is a further need for a dual mode locator that can achieved equivalent centerline and depth accuracies for both metallic and non-metallic utility lines, in real-time, as a locate technician is walking the line and marking that line with paint. Further, there is a need for line locators that can provide more permanent records of the location of the line.